A Few Facts Concerning GMT, UT, and the RGO

(original version: 3rd February 1990; this version: 3rd May 2004
This HTML version: 3rd February 2006)

In answer to the question "Does anyone know the exact difference
between GMT and UTC?" here are a few facts concerning Greenwich Mean Time,
Universal Time, and the Royal Greenwich Observatory. Various versions of this
document have been posted to Usenet newsgroups and made available on Web
sites over the years under the original title A Few Facts Concerning RGO,
GMT, and UT. The current title has been in use since the 23rd December 1995
version.

The Royal Greenwich Observatory

Prior to 1948, the observatory at Greenwich (located on a hill back from
the River Thames with a view of the London Docks) was known as the Royal
Observatory.

In 1948, the observatory moved to Herstmonceux Castle in Sussex, becoming
the Royal Greenwich Observatory (yes, even though it wasn’t at Greenwich any
more!).

The site at Greenwich became known as the Old Greenwich Observatory and
the historic buildings and instruments were progressively incorporated into
the National Maritime Museum, the main buildings of which are located at the
foot of Observatory Hill, close to the river. Highly recommended for a
visit if you’re in London!

Following the closing of the RGO in the autumn of 1998, the Old Greenwich
Observatory was renamed the Royal Observatory Greenwich
(see Where’s the RGO Now? below).

Greenwich Mean Time

Greenwich Mean Time is a time scale based on the apparent motion of the
‘mean’ sun with respect to the meridian through the Old Greenwich Observatory
(zero degrees longitude). The ‘mean’ sun is used because time based on the
actual or true apparent motion of the sun doesn’t ‘tick’ at a constant rate.
The earth’s orbit is slightly eccentric and the plane of the earth’s orbit is
inclined with respect to the equator (about 23.5 degrees) hence at different
times of the year the sun appears to move faster or slower in the sky. That’s
why an uncorrected sundial can be ‘wrong’ (if it is supposed to be telling
mean time) by up to 16 minutes. So if the mean (ie. corrected) sun is
directly over the meridian through Greenwich, it is exactly 12 noon GMT or
12:00 GMT (Prior to 1925, astronomers reckoned mean solar time from noon so
that when the mean sun was on the meridian, it was actually 00:00 GMT. This
practice arose so that astronomers wouldn’t have a change in date during a
night’s observing. Some in the astronomical community still use the pre-1925
definition of GMT in the analysis of old data although it is recommended that
the term Greenwich Mean Astronomical Time now be used to refer to time
reckoned from noon.)

Mean time on selected meridians 15 degrees apart is generally known as
standard time. For example, Eastern Standard Time (EST) is the mean solar
time of the meridian at 75 degrees W.

Universal Time

In 1928, the International Astronomical Union recommended that the time
used in the compilation of astronomical almanacs, essentially GMT, or what was
also sometimes called Greenwich Civil Time, be referred to as Universal Time
(UT). The terms Universal Time and Universal Day
were introduced at the various conferences in the 1800s held to set up the
standard time system.

There are actually a couple of variants of UT. UT as determined by
actual astronomical observations at a particular observatory is known
as UT0 (‘UT-zero’). It is affected by the motion of the
earth’s rotation pole with respect to the crust of the earth.
If UT0 is corrected for this effect, we get UT1 which is a measure of the true
angular orientation of the earth in space. However, because the earth
does not spin at exactly a constant rate, UT1 is not a uniform time
scale. The variation in UT1 is dominated by seasonal oscillations due
primarily to the exchange of angular momentum between the atmosphere
and the solid earth and seasonal tides. In an effort to derive a more
uniform time scale, scientists established UT2. UT2 is obtained from
UT1 by applying an adopted formula that approximates the seasonal
oscillations in the earth’s rotation. However, due to other
variations including those associated with the secular effects of
tidal friction (the earth’s spin is continually but gradually slowing
down), high frequency tides and winds, and the exchange of angular
momentum between the earth’s core and its shell, UT2 is also not a
uniform time scale.

So rather than base our civil time keeping on the rotation of the
earth we now use Atomic Time, time based on the extremely constant
frequency of a radio emission from cesium atoms when they change
between two particular energy states. The unit of Atomic Time is the
atomic second. 86400 atomic seconds define the length of a nominal
‘reference’ day - the length of the day as given by the earth’s
rotation around the year 1900. But because of the variations in the
earth’s spin the length of the actual day can be shorter or longer
than the nominal day of 86400 seconds. The time scale based on the
atomic second but corrected every now and again to keep it in
approximate sync with the earth’s rotation is known as UTC or
Coordinated Universal Time. The corrections show up as the leap
seconds put into UTC from time to time - usually on New Year’s Eve.
With these leap second adjustments, UTC is kept within 0.9 seconds of
UT1. Currently, the need for leap seconds is primarily due to the
effects of tidal friction. The earth’s rotation in space is monitored
by the International Earth Rotation Service (IERS) in Paris, France,
using a global network of satellite and lunar laser ranging, very long
baseline interferometry, and Navstar Global Positioning System (GPS)
stations. The IERS, in consultation with the Bureau International des
Poids et Mesures in Sèvres, France, determine when a leap second is
needed.

In 1928, when the term Universal Time was introduced, variations in
the earth’s spin were not yet known. So the term GMT was, in essence,
replaced by UT1. Despite the official adoption of the term UT, the
navigational publications of English-speaking countries retained the
term GMT as a synonym for UT1 for some time. So, even today, in
astronavigation, GMT can imply UT1. But in general usage (including
that of shortwave broadcasters such as the BBC, for example), GMT now
usually means the civil (atomic-second-based) time kept in the United
Kingdom which is the standard time of the time zone centred on the 0
degree meridian. In this (the most common) usage, the terms GMT and
UTC are identical.

But because there are two possible meanings for GMT differing by up to 0.9
seconds, the term GMT should not be used for precise purposes - particularly
not in reference to GPS observations!

The Origin of UTC

The concept of a coordinated universal time was introduced in 1960 when
the British and American national time services initiated a program to
coordinate the offsets of the frequencies and epochs (phases) of transmitted
time service radio signals from Atomic Time in approximating UT2.
Subsequently, other national time services joined the program. The BIH was
charged with the task of monitoring and maintaining the program and
introduced the term Temps Universel Coordinné or Coordinated Universal Time
for the coordinated time scale in 1964. Initially, the time scale was
derived by offsetting its rate from that of Atomic Time to agree with the
average rate of UT2 over the past year and was held fixed at that rate for
the following year. If the rate of UT2 changed significantly during the
year, then an offset (from 1962, in multiples of 100 milliseconds) could be
introduced on the first day of a month. This system of frequency and epoch
offsets was continued until 1972 when the current practice was adopted of
keeping the rate of UTC equal to that of Atomic Time and introducing leap
seconds when needed to keep UTC to within 0.9 seconds (it was 0.7 seconds
until January 1975) of UT1.

Sometimes the term World Time is used to denote UTC. This strange
and potentially confusing term (‘UTC for dummies’?) should be avoided.
Similarly, there is no clear need for the Swatch watch company’s
Internet
Time (Central European Time measured in 1/1000 of a day - a ‘beat’).

GMT and the BBC

The BBC began transmitting time signals in 1924. The chimes of Big
Ben were first broadcast at midnight beginning 1st January and on 5th
February, at the recommendation of the then Astronomer Royal, Frank
Dyson, the six pips time signal (officially known as the Greenwich
Time Signal) was inaugurated.

Control of the BBC’s six pips was taken over by the Royal Observatory
in 1949 from Abinger to where the time service had moved during the
war. The time service moved to Herstmonceux in 1957.

The time service at Herstmonceux closed down during February 1990 when
the BBC took over the generation of the six pips. Since 5th February
1990, the 66th anniversary of the start of the Greenwich Time Service,
the six pips have been synchronised to UTC by using the GPS satellite
signals which are picked up by a pair of GPS receivers on top of
Broadcasting House in London.

Where’s the RGO Now?

In March 1990, RGO officially moved from Herstmonceux Castle to the
grounds of Cambridge University’s Institute of Astronomy. On 31st
October 1998, the RGO was closed by the UK Particle Physics and
Astronomy Research Council as a cost-saving measure. Some of its
research activities have been transferred to the Royal Observatory
Edinburgh. Her Majesty’s Nautical Almanac Office was transferred to
the Rutherford Appleton Laboratory at Chilton in Oxfordshire.
With the closure of the RGO, the Old Greenwich Observatory has been
renamed the Royal Observatory Greenwich. A laser ranging station and a
GPS tracking station still operate at Herstmonceux but the castle and
estate is now owned by Queen’s University in Kingston, Ontario,
Canada, who use it as a satellite campus for their International Study
Centre. Queen’s purchased the castle in early 1993 for about
$8 million (CDN). This money, and an additional $4 million
for renovations were gifts from Dr. Alfred and Mrs. Isabel Bader of
Milwaukee, WI. Dr. Bader is a Queen’s alumnus.

To Learn More

If you would like to learn more about time you might look for the book
Greenwich Time and the Discovery of Longitude by Derek Howse originally
published in 1980 by the Oxford University Press. A second edition, titled
Greenwich Time and the Longitude: Official Millennium Guide was published
by the National Maritime Museum and Philip Wilson Publishers in 1997 (ISBN
0-85667-468-0). A special paperback edition is available exclusively from
the museum. An excellent reference on all matters concerning time is the
Explanatory Supplement to the Astronomical Almanac edited by P. Kenneth
Seidelmann of the U.S. Naval Observatory (USNO) and published by
University Science Books, Mill Valley, CA (ISBN 0-935702-68-7). There is
also a wealth of information on time at USNO’s
Directorate of
Time Web site. For information on Queen’s University’s
International Study Centre at Herstmonceux Castle, visit their
Web site.
A GPS World article discussing the future of the leap second can be found
here.